Studies will be made on purification of peroxidase using antibodies cross-reacting with thyroid iodide peroxidase. These antibodies will also be used to localize the specific iodide peroxidase in the thyroid cell, for an immune assay of the enzyme, and for studies of turnover of the enzyme in response to alterations of thyroid function. H2O2 supply in the thyroid will be studied by development of antibodies to microsomal NADH-cytochrome c reductase, using techniques applied in our recent study of NADPH-cytchrome c reductase. Control of thyroid hormonogenesis will be investigated by measuring iodide trapping, iodide binding, H2O2 generation, reductase activity, and thyroglobulin synthesis at several levels of thyroid activation in experimental animals. Binding of T3 to rat liver nuclei will be investigated in vivo and in isolated nuclei in vitro. The binding protein will be purified by affinity chromatography and its role, if any in regulation of DNA transcription will be evaluated.